mapReduce.js

// RDB - select birth , working , count(*) from users group by birth , working

// mongoDB map reduce 

db.users.group({
    key:{birth:1,working:1},
    reduce:function(obj,prev){
        if(!obj.hasOwnProperty("key")){
            prev.birth = obj.birth;
            prev.working = obj.working;
        }
    },
    initial:{}
})